Moving formant band-pass amplifier for an electronic musical instrument

ABSTRACT

A circuit for achieving the moving or changing formant effect in an electronic musical instrument such as an electric organ. The circuit employs a band-pass amplifier with parallel T resistivecapacitive feedback circuits in which an FET (Field Effect Transistor) is employed as a variable resistance to cut in, by degrees, one of the T feedback circuits. The effective resistance of the FET and thus the band-pass of the amplifier are varied by a capacitive and resistive timing circuit that is controlled via a keying circuit from one or more keys or other player-operated controls, such as the manual keys of an electric organ.

United States Patent Donald R. Kern Buffalo Grove, Ill. 749,334

Aug. 1, 1968 Mar. 9, 1971 Hammond Corporation Chicago, Ill.

[72] Inventor [21 Appl. No. [22] Filed [45] Patented [73] Assignee [54]MOVING FORMANT BAND-PASS AMPLIFIER FOR AN ELECTRONIC MUSICAL INSTRUMENT11 Claims, 2 Drawing Figs.

[52] U.S.Cl 84/l.19, 84/1.25,84/1.11,330/86,330/l03 [51] lnt.Cl GlOh1/02 [50] Field olSearch 84/l.01, 1.1 l, 1.19, 1.24, 1.25, (F), (FB),(B); 330/85, 103, 86

[5 6] References Cited UNITED STATES PATENTS 2,927,282 3/1960 Gardberg331/142 2,971,161 2/1961 Clevenger 330/85 3,205,294 9/1965 Maynard84/1.11 3,433,937 3/1969 McCarthy 330/103 2,245,365 6/1941 Riddle84/(FB) 3,379,820 4/1968 Olson 84/1.24

Primary Examiner-Milton O. Hirshfield Assistant Examiner-Stanley J.Witkowski Attorney-Gradolph, Love, Rogers & Van Sciver ABSTRACT: Acircuit for achieving the moving or changing formant effect in anelectronic musical instrument such as an electric organ. The circuitemploys a band-pass amplifier with parallel T resistive-capacitivefeedback circuits in which an FET (Field Effect Transistor) is employedas a variable resistance to cut in, by degrees, one of the T feedbackcircuits. The effective resistance of the FET and thus the band-pass ofthe amplifier are varied by a capacitive and resistive timing circuitthat is controlled via a keying circuit from one or more keys or otherplayer-operated controls, such as the manual keys of an electric organ.

BRIG/l7 Bus PATENIED HAR 91911 SHEET 1 UF 2 PATENTED MAR 9 I87! SHEET EOF 2 MOVING FORMANT BAND-PASS AMPLIFIER FOR AN ELECTRONIC MUSICALINSTRUMENT FIELD OF THE INVENTION Formanting circuits for electricalmusical instruments.

DESCRIPTION OF THE PRIOR ART While some attempts of achieving movingformant effects, such as those disclosed in the US. Pat. to R. H.Peterson; No. 3,316,341, entitled Electrical Musical Instruments," whichissued on Apr. 25, 1967, have been attempted before, these usuallyinvolve expensive constructions or accomplish limited effects which maybe generally similar to those presently proposed.

SUMMARY OF THE INVENTION The present invention has as its general objectthe provision of a moving formant device of a new and improvedconstruction. A more specific object of the invention is the provisionof such a device that is both effective in use and economical to makeand maintain. One object of the present invention is the provision of amoving formant device which may affect only a portion of the note voicedwhile leaving the other portion unaffected.

The features of the present invention which are believed to be novel areset forth with particularity in the appended claims. The invention,together with further objects and advantages thereof, may best beunderstood by reference to the following description taken in connectionwith the accompanying drawings, in the several FIGS. of which likereference numerals identify like elements, and in which:

FIG. 1. is an electric circuit diagram of a moving formant deviceconstructed in accordance with the principles of the present invention;and

FIG. 2 is an electric circuit diagram of another moving formant deviceconstructed in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1 the devicethere depicted is generally designated by the numeral and forms part ofan electrical musical instrument of the electric organ type. The organincludes a bright bus 11 from which an electrical musical tone may betaken. Playing key contacts or keyers, not shown since they may beconventional, will key the proper signals to the bus 11 as particularplaying keys areactuated. A selector switch 12 couples the output of busbar 11 to a band-pass amplifier generally designated 9.

The amplifier 9 may be consideredto be a two transistor parallel Tfeedback oscillator in which one of the feedbacks T has been slightlydetuned, thus preventing oscillation but instead resulting in a resonantpeak in the audio spectrum at approximately the oscillator frequency.

The switch 12 is directly connected to one side of a capacitor 13. Aresistance 14 is connected from that side of capacitor 13 to a plane ofreference potentialor ground. The other side of capacitor 13 isconnected through a resistor 15 to the base of a first transistor 16 ofthe NPN type. Also connected to the base of transistor 16 is one end ofa resistor 17, whose other end is grounded, and one end of a resistor 18whose other end is connected to a source of direct current biasingpotential B+.

The emitter of the first transistor 16 is connected to the base of asecond NPN transistor 19. A resistor 20 is connected from theinterconnected emitter of transistor 16 and base of transistor 19 toground. The collectors of both transistors 16 and 19 are connectedtogether and through a resistor 21 to the voltage source 13+. Theemitter of transistor 19 is connected through a resistor 22 and aparallel connected capacitor 23 and resistor 24 to ground.

The collectors of the transistors 16 and 19 are connected together andthrough a direct current blocking capacitor 25 to an output terminal 26,connected in turn to conventional musical instrument amplifiers andmixing circuits which supply signals at an appropriate level to one ormore speakers 71. The terminal 26 also is connected via a line 27 to afeedback network generally designated 28. The network 28 feeds back theoutput at terminal 26, via a line 29 and through a capacitor 30, to thebase of the transistor 16. The circuit 28 includes a first T network ofa pair of series connected resistors 31 and 32 whose joined ends areconnected through a capacitor 33 to ground and whose free ends areconnected respectively to the lines 27 and 29. The circuit 28 alsoincludes a second T network comprising a pair of series connectedcapacitors 34 and 35 whose common ends are connected through a variableresistance 36 to ground. The free end of capacitor 35 is connected toline 29 and also through another capacitor 37 to ground. The free end ofcapacitor 34 is connected through a resistor 38 to ground.

Connected between the junction of the free end of the capacitor 34 andthe resistor 38 and the line 27 is a Field-Effect Transistor (FET) 40 ofthe p-channel type. The FET is a well known device now in commercialuse. Those unfamiliar with its characteristics are referred to "FieldEffect Transistors," by L. .I. Sevin, Jr., McGraw, Hill, 1965. Referencealso may be made to Application Note AN-2l l of Motorola SemiconductorProducts, Inc. entitled Field Effect Transistors in Theory and Practice"by J F. Kane et al., dated Sept. 1966. The drain of the FET 40 isconnected to line 27 and the source of FET 40 is connected to the seriesjunction of capacitor 34 and resistor 38. The gate of the FET 40 isconnected to the movable tap of a variableresistor 41 which has one endgrounded and the other end connected to one end of a fixed resistor 42.The junction between these resistors is connected to ground through atiming capacitor 43. The other end of the fixed resistor 42 is connectedthrough the collector emitter circuit of a transistor 45 to ground andthrough a resistor 46 to 8+.

The transistor 45 is of NPN type, has its emitter grounded and has itsbase connected to one contact 44 of a single pole, single throw selectorswitch 47, the other side of which is grounded. When the switch 47 isopen, the bias of base of transistor 45 is determined by a keyingcircuit generally designated by the numeral 50.

The keying circuit 50 may be anything suitable. but as shown consists ofa circuit from the base of transistor 45 through a current limitingresistor 48 to a pair of NPN transistors 51 and 52. The transistor 51has its collector connected to the resistor 48 and also to the biassource 13+ through a resistor 53. Its emitter is grounded and the baseis connected to the collector of the transistor 52 and through aresistor 54 to the bias source B+.

The base of the transistor 52 is connected to the series junction of apair of resistors 55 and 56 which are connected in order between thebias source 8+ and ground. In parallel connection with the resistor 56is a diode 57 oriented to pass current from the base of transistor 52 toground.

The emitter of transistor 52 is connected to ground through a diode 58and resistor 59in parallel. The diode 58 is oriented to allow currentflow only from the emitter of transistor 52 to ground. a

The emitter of transistor 52 is also connected to one side of a normallyopen control'switch 60 which typically will be closed by actuation ofthe organ playing keys. The other side of the switch 60 is connected,through a current limiting resistor 61 to a source of positivepotential, designated B'+ at above 4v.

The biasing for the transistors 16 and 19 as established by the voltagesource 8+ and the resistors associated with the transistors, is such asto cause them to operate in class A amplification. The FET is similarlybiased as to operate as a variable resistance between its drain andsource. I

In overall operation the device 10 may deliver to the amplifying andm'iiting circuit 70 a musical tone signal which exhibits the movingformant effect in response to the closing of the key switch 60. With theselector switch 47 closed, so that the contact 44 is grounded, thecircuit does not respond to the key contacts 60, but delivers a musicaltone signal at its output terminal 26 which approximates a trumpet notein its character and does not exhibit a moving formant effect.

The signal from the bright bus 11 is transferred through the capacitor13 and resistor 15 to the base of the transistor 16. The transistors 16and 19 amplify this signal and supply it to the output terminal 26through the capacitor 25. The feedback circuit 28 has two feedbackpaths, one via line 27, the T circuit of resistors 31 and 32 and line29; and another through the drain-source circuit of the FET 40, thecapacitors 34 and 35 and line 29. The effective resistance of the drainsource circuit of the FET 40 controls or modulates the signal thatpasses through this second path. In effect, therefore, the feedbackcircuit is an RC twin T-frequency rejection network which has been splitsuch that the output signal of transistor 19 is applied to half of thenetwork (31-32-33) while a variable voltage, but otherwise similarsignal, is applied to the other network half (34-35-36). The rejectionfrequency, therefore, varies depending upon conduction through the FET.

The amplifier 9 is, therefore, of the band-pass type and has itsband-pass determined by the capacitive resistive feedback circuit 28.When the efiective resistance of drain source circuit of the FET 40 ishigh, the second feedback path is effectively cut out of the circuit andthe band-pass of the amplifier 9 is centered about one high resonancepeak. When the drain source resistance of the FET 40 is low, thealternate path is fully open and the band-pass of the amplifier 9 iscentered about a low resonance peak.

The movement of the band-pass of the amplifier 9 from its low to highresonant peak is thus governed by the effective drain source resistanceof the FET 40 which resistance is in turn governed by the direct currentbias on its gate. The voltage on the gate is determined by the timingcircuit of the resistances 41 and 42 and the capacitor 43.

Normally, when the transistor 45 is saturated, that is, when theresistance of the collector-emitter circuit of the transistor 45 is low,the junction of the resistors 42 and 46 is effectively at groundpotential and thus the gate of the FET 40 is also at ground potential.This keeps the FET 40 source drain resistance low and the band-pass ofthe amplifier 9 low. However, when the transistor 45 is renderednonconductive, current flows from the 13+ source through the resistors46 and 42 and to the parallel capacitor 43 and resistor 41 circuit. Werethe timing capacitor 43 not in this circuit the voltage at the top ofthe resistor 41 would jump almost immediately to a level determined bythe voltage dividing resistors 41, 42 and 46. However, as the capacitoris in the circuit the current is diverted initially into charging thatcapacitor. As the charge on the capacitor 43 rises, the potential at thecenter tap of resistor 41 rises and the potential change at the FET gateis substantially as depicted by the curve 72 on the small graph of FIG.1.

If after approximately reaching its ultimate value, at which time thecapacitor 43 is fully charged, the transistor 45 is again renderedconductive, the capacitor 43 discharges through resistor 42 as well asthrough permanently connected resistor 41. If the value of the resistor42 is chosen to be small in comparison to that of resistor 41 a fastdecline of the voltage at the gate of the FET 40 will be achieved,approximating the curve 73 of the small graph of FIG. 1.

The transistor 45 is rendered nonconductive by grounding its base. Thismay be done by the trumpet selector switch 47 or by the keying circuit50 in response to closure of the key contacts 60. When done by theswitch 47, the band-pass of the amplifier is fixed at its high peak andthe tone signal produced at output terminal 26 is like a trumpet anddoes not have the moving formant effect. However, when switch 47 isopen, the band-pass amplifier 9 will normally be at its low band-passpeak and a muted trumpet sound electrical signal will be translatedthrough the amplifier 9 to the output terminal 26. When the key contacts60 are closed, the potential at the emitter of the transistor 52 iscaused to rise to the threshold value of diode 58, cutting off thattransistor and turning on the transistor 51. This causes the voltage atthe collector of transistor 51 to fall from approximate B+ to groundwhich in turn effectively grounds the base of the transistor 45. Whenthis occurs the voltage at the gate of the FET rises as the curve 72 andthe band-pass of the amplifier 9 goes from its low peak (mute) to itshigh peak (trumpet) resulting in the moving formant effect simulatingopening a muted trumpet.

For purposes of definiteness of this disclosure and not for limitation,the following component values are submitted. It will be understood bythose skilled in this art that numerous variations in values, as well asreorientation of components and substitution of equivalents for elementsor groups of elements are possible and that these are contemplated to bewithin the scope of the present invention.

Resistors: Ohm

15 680K 17, 53 K 18, 48 470K 20, 21 10K 22 a K 24 2.7K

6 0-100K 38, 42 6.8K 41 0-1000K 46 270K 54 t 220K 55 12K 56, 61 1K 59Capacitors: Mf.

Transistor 16 is of the 2N339lA type, transistors 19, 45, 51 and 52 areof the 2N3393 type. The FET 40 is of 2N3819 type.

The diodes 57 and 58 are of typical silicon junction type and are usedto establish a potential at their threshold value of about 0.6V. Theyaid in temperature compensating the circuit.

The impressed signal from the bright bus bar 11 is normally at about10.7v. peak-to-peak.

The components were chosen to provide a formant movement of about l/aoctaves, with the high resonant peak providing a formant similar tothe normal 8' trumpet stop on a typical electrical organ.

Referring to FIG. 2, an alternative circuit for the moving formantdevice is there depicted and is similarly generally indicated by thenumeral 10'. Elements of this embodiment which are similar to those ofthe previous embodiment will be identified with primes of the samenumbers, it being understood that they are connected as in thepreviously described embodiment unless noted to the contrary. The valuesof the otherwise similar components may be different, however.

The device 10 includes an amplifier 9 generally similar to the amplifier9 of the previous embodiment, except that it includes an NPN transistor80 in place of the FET 40 and a parallel inductor 81 and capacitor 82 inplace of the load resistor 2].

The resistance of the emitter-collector circuit of transistor 80 iscontrolled by either a simplified keying circuit 83 or by alow-frequency oscillator circuit 84, either of which may be connected tothe base of transistor 80, depending upon the position of a selectorswitch 85.

More specifically, the amplifier 9 includes an input terminal 86 whichis connected to one end of a resistor 15' whose other end is connectedto the base of an NPN transistor 16'. This base also is connectedthrough a resistor 18 to B+ and through a resistor 17' to ground. Theemitter of the transistor 16' is connected to the base of an NPNtransistor 19 and the collectors of both transistors 16' and 19' areconnected together and through a parallel capacitor 82-inductor 81circuit to B+, and also to a signal output terminal 26 and to a feedbackcircuit 28 via a line 27. The emitter of transistor 19 is connected toground through a parallel connected resistor 24' and capacitor 23'. Thecircuit 28 comprises a pair of T circuits, one of resistors 31', 32 andcapacitor 33 and the other of capacitors 34, 35' and variable resistor36, the latter being connected to lead 37 by way of the collectoremittercircuit of a transistor 80. The other side of circuit 28' is connectedto the base of transistor 16 through a line 29' and a capacitor 30.

The collector of transistor 80 is connected to lead 27 and a capacitor87 is connected between the collector and its base. The base is alsoconnected through a resistor 46 to B+ and through a resistor 41' toground and through a resistor 88 to the blade ofthe previously mentionedswitch 85. The switch blade is also connected to ground through a timingcapacitor 43'.

The keying circuit 83, by way of a line 89, is connected to one of twocontacts that the blade of switch 85 may engage. Line 89 is connected tothe collector of a transistor 90 and also through a resistor 91 to B+.The emitter of the transistor 90 is coupled through a resistor 92 toground, and the base of that transistor is connected through a resistor93 to B'+ and through a resistor 94 to ground. The base of transistor 90is also connected through a current limiting resistor 95 to B'+ by wayof key actuated contacts 60.

The low-frequency frequency oscillator 84 which runs at vibrato rate,about 6.5 H includes a pair of NPN transistors 96 and 97 whosecollectors are connected in common and,

through a resistor 98, to 3+ and through a series connected capacitor 99and variable resistance 100 to the other terminal of the switch 85.

The emitter of transistor 96 is connected through a resistor 101 toground while the emitter of transistor 97 is connected to the base oftransistor 96. The base of transistor 97 is connected through a resistor102 to its collector and through a resistor 103 to ground. The base isalso connected to one side of a capacitor 104, the other side of whichis connected, through a variable resistor .105, to ground and to oneside of another capacitor 106. The other side of this capacitor 106 isconnected through another variable resistor 107 to ground and to oneside of yet another capacitor 108. The other side of this capacitor isconnected to the collectors of the transistors 96 and 97.

The variable resistors are mechanically ganged and manually controlledto vary the oscillation rate. The variable resistance 100 varies theswing limits or the degree of movement per oscillation when theoscillator 84 drives the moving formant of the amplifier 9.

Basically, in operation, the device when the blade of the selectorswitch 85 is connected to the oscillator circuit 84, functions to movethe formant signal transferred through the amplifier 9 cyclically at arate determined by the oscillator 84, for example at 6.5 to 7 cycles persecond. This yields a pleasing musical effect which is somewhat like avibrato.

When the switch 85 has its blade connected to line 89, it functions muchas the embodiment of FIG. 1 when the switch 47 of thatembodiment isopen. That is, a formant movement is produced in response to the closureof the key actuated switch *60.

In the circuit of FIG. 2 the following values may be employed for thevarious components. These specific values are,

of course, suggested by way of example and for completeness ofdisclosure. It should be borne in mind, however, that nution andarrangements can be successfully used.

Resistors: Ohm

103 2000K 01000K 46 1000K 105, 107 0-20OK 18 K 15, 88, 101 100K 91 68K93 56K 31, 32 27K 36 0-25K 17', 41' 22K 38, 94 10K 98 5. 6K 102 4. 7K 952. 7K 92 1. 2K 24' 1K Capacitors: Mf.

Inductor:

1 2300 T, Ferrite Cup Core.

The bias source B+ is 15 volts, while that of B'+ is 4 volts aspreviously stated.

This device 10 as shown and described effectively moves a formantslightly more than two octaves either under control of the playing keysor optionally under the influence of a repetitive voltage sources suchas the vibrato oscillator. The trumpet stop of an organ played throughthis device under key control sounds like a mute trumpet being opened upas the playing key is held.

One of the effective uses of the device 10 or 10' is in keying it with atouch responsive system on the upper manual and with the bright wavevoices fed through it. When this device is driven at the vibrato rate,it sounds very much like vibrato on the frequencies in the formantrange. Since the formant range with the components as shown in FIG. 2and values as detailed above, is from 400 cycles to l900 cycles, it ispossible to play low-frequency bright wave tones in which the higherfrequencies appear to have vibrato without the lower frequencies beingaffected. With proper selection of formant range, this device iseffective to provide rnanual selection of vibrato within a limitedfrequency range on organs that normally have only unitary vibrato.

It is apparent that this invention provides a new and improved movingformant device that is versatile and capable of several applications inelectrical musical instruments. Furthermore, it is relativelyinexpensive as it employs low cost components, while effectivelyaccomplishing its intended ends in an efficient manner.

Although the two embodiments described in detail use a transistor or FETas a voltage responsive variable resistor to control the effectivenessof one of the two feedback paths, it

' will be appreciated that there are other voltage responsive claims isto cover all such changes and modifications as fall within the truespirit and scope of the invention.

I claim:

1. A musical instrument moving formant devicecomprising an audiofrequency amplifier having a music signal input and an amplified output,means providing a feedback :path connected forzfeeding back signal fromsaid output to saidtinput,

said feedback path consisting of an RC, parallel T, frequency rejectioncircuit, one of the parallel branches of said circuit comprising a pairof resistors in series with a center connected capacitor and the otherof the parallel branches comprising a pair of series connectedcapacitors with a center connected resistor. a voltage dependentvariable impedance incorporated in one only of said branches andconnected so that conduction through the last said branch from saidoutput to said input depends upon the voltage applied to said voltagede- V pendent variable impedance, and means connected for applying avariable control voltage to said variable impedance to vary theconduction through one only of the parallel branches as a function ofsaid control voltage.

2. The moving formant device as called for in claim 1 in which the meansfor applying the variable control voltage comprises a timing circuit ofthe type for providing a-gradual change of voltage for a few secondsfollowed by a steady voltage when activated and a rapid restoration ofvoltage to the original level when subsequently deactivated, and amanual control connected for activating and deactivating said timingcircuit.

3. The moving formant device as called for in claim 1 in which thevoltage dependent variable impedance device is a field effecttransistor.

4. The moving formant device as called for in claim 1 in which thevariable impedance device is incorporated in the branch which has thetwo capacitors in series and a center connected resistor.

5. The moving formant device as called for in claim 1 in which therejection frequency of the parallel T circuit is lower when theimpedance of said variable impedance is low than it is when theimpedance of said variable impedance is high.

6. The moving formant device as called for in claim 5 in which thecontrol voltage is applied to reduce the impedance of the variableimpedance device for the duration of the voltage.

7. The moving formant device as called for in claim 1 in which the meansfor applying a variable control voltage is an oscillator operating at avibrato frequency.

8. The moving formant device as called for in claim 2 in which themanual control is any of a group of playing keys of an electric organ.

9. The moving formant device as called for in claim 6 in which the meansfor applying the variable control voltage comprises a timing circuit ofthe type which provides a gradual voltage change as a function of timefor a few seconds followed by a steady state voltage when actuated and arapid restoration to the original state when subsequently deactivated,and manual means connected for activating and deactivating said timingcircuit.

10. The moving formant device of claim 9 in which said manual means isany of a group of playing keys of an electric organ.

11. The moving formant device as called for in claim 9 in which thevoltage dependent variable impedance is a field effect transistor.

{22 55 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3, 569,603 Dated March 9 1971 Inventor(s) Donald R. Kern It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Second paragraph of Abstract omitted, and should be inserted on top pageof patent after first paragraph:

-A second embodiment of the moving formant circuit employs a transistorin place of the FET, an: inductive-capacitive load for the amplifierthat com sates for the frequency shift of the amplifier s out to producea generally constant subjective tone ampl: tude therefrom, and analternate circuit for activat: the formant change comprising a variablelow frequem oscillator. Driving the formant changing transistor theoscillator operating at vibrato speeds results i1 a musical effectsimilar to vibrato.--

Col. 2, line 67, "above" should read -abo1 Col. 5, line 14, "37" shouldbe --27--.

Claim 11, "as called for in claim 9" shoul read --as called for in claim10 Signed and sealed this 19th day of October 1971.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOI'TSGHALK Attesting Officer ActingCommissioner of P

1. A musical instrument moving formant device comprising an audiofrequency amplifier having a music signal input and an amplified output,means providing a feedback path connected for feeding back signal fromsaid output to said input, said feedback path consisting of an RC,parallel T, frequency rejection circuit, one of the parallel branches ofsaid circuit comprising a pair of resistors in series with a centerconnected capacitor and the other of the parallel branches comprising apair of series connected capacitors with a center connected resistor, avoltage dependent variable impedance incorporated in one only of saidbranches and connected so that conduction throuGh the last said branchfrom said output to said input depends upon the voltage applied to saidvoltage dependent variable impedance, and means connected for applying avariable control voltage to said variable impedance to vary theconduction through one only of the parallel branches as a function ofsaid control voltage.
 2. The moving formant device as called for inclaim 1 in which the means for applying the variable control voltagecomprises a timing circuit of the type for providing a gradual change ofvoltage for a few seconds followed by a steady voltage when activatedand a rapid restoration of voltage to the original level whensubsequently deactivated, and a manual control connected for activatingand deactivating said timing circuit.
 3. The moving formant device ascalled for in claim 1 in which the voltage dependent variable impedancedevice is a field effect transistor.
 4. The moving formant device ascalled for in claim 1 in which the variable impedance device isincorporated in the branch which has the two capacitors in series and acenter connected resistor.
 5. The moving formant device as called for inclaim 1 in which the rejection frequency of the parallel T circuit islower when the impedance of said variable impedance is low than it iswhen the impedance of said variable impedance is high.
 6. The movingformant device as called for in claim 5 in which the control voltage isapplied to reduce the impedance of the variable impedance device for theduration of the voltage.
 7. The moving formant device as called for inclaim 1 in which the means for applying a variable control voltage is anoscillator operating at a vibrato frequency.
 8. The moving formantdevice as called for in claim 2 in which the manual control is any of agroup of playing keys of an electric organ.
 9. The moving formant deviceas called for in claim 6 in which the means for applying the variablecontrol voltage comprises a timing circuit of the type which provides agradual voltage change as a function of time for a few seconds followedby a steady state voltage when actuated and a rapid restoration to theoriginal state when subsequently deactivated, and manual means connectedfor activating and deactivating said timing circuit.
 10. The movingformant device of claim 9 in which said manual means is any of a groupof playing keys of an electric organ.
 11. The moving formant device ascalled for in claim 9 in which the voltage dependent variable impedanceis a field effect transistor.